1. Field of the Invention
The field of the present invention relates laser scanners such as used in laser eye surgery or other applications, and more particularly to procedures for incising the cornea using a laser, and systems for making such incisions, during ophthalmic surgery.
2. Description of Related Art
Laser Assisted In-Situ Keratomileusis (LASIK) and other ophthalmic surgical procedures involve forming a flap of corneal tissue, which is separated from the cornea and folded back to expose underlying stromal tissue. The stromal tissue is then reshaped to correct for conditions such as near-sightedness or astigmatism using a pulsed laser. The laser emits pulses at a known frequency, and each pulse photoalters tissue at the focal point of the laser beam. The focal point of the laser beam is swept over the stromal tissue in a scan pattern, such as a raster pattern, under computer control until the stroma is reshaped as desired. The flap is then folded back over the stroma, to which it becomes reattached during the healing process.
The flap may be cut using a microkeratome, which is a precision surgical instrument with an oscillating blade. In the alternative, the flap may be created using a pattern of laser pulses. To create the corneal flap using a laser, two steps are performed. In one step, a sidecut is created around a desired perimeter of the flap. Both ends of the sidecut terminate without intersecting, thereby leaving an uncut segment that later serves as a hinge for the corneal flap. In another step, the flap is separated from the underlying stromal tissue by scanning the laser focal point over an area called the “resection bed,” the perimeter of which is approximately defined by, and slightly larger than, the sidecut. Once the sidecut and the resection bed are made, then the flap can be lifted and folded back to reveal the stromal tissue for reshaping. Suitable surgical equipment for creating the corneal flap using a laser is known in the art.
Laser scanners for ophthalmic surgical systems generally utilize a pair of scanning mirrors or other optics to angularly deflect and scan the laser beam. Scanning mirrors driven by galvanometers may be employed, each scanning the laser along one of two orthogonal axes. A focusing objective, whether one lens or several lenses, images the laser beam onto a focal plane of the optical system. The focal point of the laser beam may thus be scanned in two dimensions (x and y) within the focal plane of the optical system. Scanning along the third dimension, i.e., moving the focal plane along the optical axis (z-axis), may be achieved by moving the focusing objective, or one or more lenses within the focusing objective, along the optical axis. In preparing a corneal bed for flap separation, for example, a circular area may be scanned using a raster pattern driven by the scanning mirrors. The laser photoalters the stromal tissue by scanning the focal point of the laser in a pattern of spots, the distribution of which is determined by the pulse frequency, the scan rate, and the amount of scan line separation.
Generally, higher scan rates, i.e., the step rate at which the focal point of the laser is moved, enable shorter surgical times by increasing the rate at which corneal tissue can be photoaltered. Shorter surgical times are less stressful for the patient, and may reduce the likelihood of errors introduced by excessive movement of the patient. As scan rates increase, greater demands are placed on the laser scanner used to direct the laser beam. Laser scanners used to control the scanning motion may begin to introduce mechanical lag errors in focal point positioning at higher scan rates. It is desirable to correct these errors, without requiring potentially costly changes to laser scanner hardware.